\tightlist was introduced with pandoc 1.14 (see this commit). The most probable cause for this error is that you are using a template from a previous version and did not update it. This can be fixed by adding a couple of lines to your latex template. They can be found at $DATA-DIR/templates/default.latex for regular latex and $DATA-DIR/templates/default.beamer for beamer. You'll need to add those two lines (before the \begin{document}):